There is a defect detection method using an electron image generated by a scanning electron microscope for pattern inspection of a wafer in a manufacturing process of a semiconductor integrated circuit or for pattern inspection of a photomask for forming the pattern.
In a method using an electron image generated by a scanning electron microscope, a specimen is two-dimensionally irradiated with an electron beam, and secondary electrons or backscattered electrons are detected to create a two-dimensional image. In order to increase an S/N ratio of the obtained image, the specimen is scanned with an electron beam multiple times, and a plurality of detection signals obtained from the same position are averaged.
A dielectric material, such as resist, oxide film, or the like, is used in a semiconductor integrated circuit which is an example of a specimen. In addition, in order to achieve high throughput inspection, a specimen may be irradiated with a large current electron beam. For these reasons, the specimen is charged and as a result, the electron beam is directed to a position different from a target position. If such a phenomenon occurs, an electron-beam irradiating position in each scan changes while the specimen is scanned with the electron beam multiple times. In this specification, this phenomenon is referred to as image drift. When the image drift occurs, a clear image cannot be obtained by the averaging process described above.
FIGS. 10A to 10C are diagrams showing an example of a phenomenon in which an image becomes unclear due to the image drift. More specifically, FIG. 10A is a schematic diagram showing a manner in which the electron beam scans a surface of a specimen 210 twice. The specimen 210 includes a pattern 200 formed thereon. As shown in FIG. 10A, the electron-beam irradiating positions in the second scan are slightly shifted from those in the first scan. This is because, as described above, the dielectric material forming the surface of the specimen is charged by the electron beam irradiation in the first scan.
FIG. 10B is a schematic diagram showing a pattern image 201 obtained by the first scan and a pattern image 202 obtained by the second scan. As can be seen from FIG. 10B, the position of the pattern image 202 is slightly shifted from the pattern image 201. FIG. 10C is a schematic diagram showing an average image 203 obtained by averaging the two pattern images 201, 202 shown in FIG. 10B. As a result of the positional shift between the pattern images 201, 202, the obtained average image 203 becomes unclear.